Obesity - together with its co-morbidities insulin resistance, type-2 diabetes and cardiovascular disease - is one of America's major health challenges in the twenty-first century. One crucial link between obesity and downstream cardio-metabolic disorders is chronic low-grade inflammation of visceral adipose tissue, and ultimately systemically. Strikingly, macrophages can constitute as many as half of the cells residing in the visceral fat of obese humans and mouse models, and this cell-type is recognized to be one of the critical drivers of obesity-associated inflammation. It was recently reported that a substantial population of Foxp3? regulatory T cells (Tregs) resides in the visceral adipose tissue of lean, but not obese, mice (and humans). This population has a distinct gene-expression profile and T cell receptor (TCR) repertoire. Loss- and gain-of-function experiments established that Tregs are capable of regulating adipose-tissue inflammation and systemic metabolic indices, promoting insulin resistance, and thereby protecting from type-2 diabetes. The overall goal of this proposed project is to elucidate the generation, dynamics and function of visceral- fat Tregs. Specifically, we aim to: 1. Determine the origin of Tregs residing in visceral adipose tissue, utilizing a combination of approaches entailing cell transfers, imaging of cells photo-tagged in vivo, and construction and characterization of novel reporter and TCR-transgenic mouse lines. The role of candidate molecules in migration of visceral-fat Tregs will then be addressed. 2. Establish the role of PPARg in the emergence and function of fat-resident Tregs. The activities of this nuclear receptor super-family member -- highly and specifically induced in visceral-fat Tregs -- will be explored through functional experiments and gene-expression profiling on cells from mice lacking PPARg specifically in Tregs, mice with a greatly and specifically expanded visceral-fat Treg population, and ex vivo naive T cells retrovirally transduced with foxp3 pparg. 3. Define elements underlying the impressive reduction in Tregs in visceral adipose tissue of obese mice, addressing, in particular, whether their migration is blocked, whether their survival in the adipose tissue niche is compromised, and what pathways and molecules are implicated. Results from these studies should greatly enrich our understanding of how adipose tissue inflammation is regulated in the lean state and dysregulated with the onset of obesity, likely revealing novel therapeutic targets. Given increasing recognition of the unfortunate side-effects of the front-line diabetes-dampening thiazolidinedione (TZD) drugs, there is currently great interest in the discovery of new targets capable of protecting from or reducing insulin resistance and downstream cardio-metabolic diseases.